FISH (fluorescence in situ hybridization) is a cytogenetic technique that is used to detect and localize the presence or absence of specific DNA sequences on chromosomes. FISH uses fluorescent probes that bind to only those parts of the chromosome with which they show a high degree of sequence complementarity. Fluorescence microscopy can be used to find out where the fluorescent probe is bound to the chromosomes. FISH is often used for finding specific features in DNA for use in genetic counseling, medicine, and species identification. FISH can also be used to detect and localize specific RNA targets (mRNA, lncRNA and miRNA) in cells, circulating tumor cells, and tissue samples. In this context, it can help define the spatial-temporal patterns of gene expression within cells and tissues.
Human genomic DNA is a mixture of unique sequences and repetitive sequences that are present in multiple copies throughout the genome. In some applications, it is desirable to generate hybridization probes that anneal only to unique sequences of interest on a chromosome. Preparation of unique sequence probes is confounded by the presence of numerous classes of repetitive sequences throughout the genome of the organism (Hood et al., Molecular Biology of Eucaryotic Cells (Benjamin/Cummings Publishing Company, Menlo Park, Calif. 1975). The presence of repetitive sequences in hybridization probes reduces the specificity of the probes because portions of the probe bind to other repetitive sequences found outside the sequence of interest. Thus, to ensure binding of hybridization probes to a specific sequence of interest, probes lacking repetitive sequences are needed.
Recent contributions have addressed this question by inhibiting hybridization of the repetitive sequences with the use of unlabeled blocking nucleic acids (U.S. Pat. No. 5,447,841 and U.S. Pat. No. 6,596,479). Use of blocking nucleic acids in hybridizations is expensive, does not completely prevent hybridization of the repetitive sequences, and can distort genomic hybridization patterns (Newkirk et al., “Distortion of quantitative genomic and expression hybridization by Cot-1 DNA: mitigation of this effect,” Nucleic Acids Res. vol 33 (22):el91 (2005)). Thus, methods that prevent hybridization of repeat sequences without the use of blocking DNA are desirable for optimal hybridization.
One means to achieve this is to remove unwanted repeat segments from the hybridization probes prior to hybridization. Techniques involving the removal of highly repetitive sequences have been previously described. Absorbents, like hydroxyapatite, provide a means to remove highly repetitive sequences from extracted DNA. Hyroxyapatite chromatography fractionates DNA on the basis of duplex re-association conditions, such as temperature, salt concentration, or other stringencies. This procedure is cumbersome and varies with different sequences. Repeat DNA can also be removed by hybridization to immobilized DNA (Brison et al., “General Methods for Cloning Amplified DNA by Differential Screening with Genomic Probes,” Molecular and Cellular Biology, Vol. 2, pp. 578-587 (1982)). In all of these procedures, the physical removal of the repetitive sequences will depend upon the strict optimization of conditions with inherent variations based upon the base composition of the DNA sequence.
Several other methods to remove repetitive sequences from hybridization probes have been described. One method involves using a cross-linking agent to cross-link repetitive sequences either to directly prevent hybridization of repetitive sequences or to prevent amplification of repeat sequences in a PCR reaction. (U.S. Pat. No. 6,406,850). Another method uses PCR assisted affinity chromatography to remove repeats from hybridization probes (U.S. Pat. No. 6,569,621). Both of these methods rely on the use of labeled DNA to remove repeat sequences which makes these processes complex and difficult to reproduce. Further, both methods are time consuming, requiring multiple rounds of repeat removal to produce functional probes suitable for use in fluorescent in situ hybridization (FISH) or other hybridization reactions requiring high target specificity.
Thus, methods for removing repetitive sequences from probes are desired.